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Glossary of Genetic Terms for Articles on the Double Helix Ranch Web Site

Allele: A particular form of a gene, composed of a particular sequence of nucleotides. Every diploid organism (such as a cow or bull) inherits two copies of every gene (one from each parent). These two copies may be identical (i.e., represent the same allele) or may be different (i.e., represent different alleles).

Amino acids: The building blocks of proteins. There are 20 common amino acids that make up virtually all proteins.

Artificial selection: Differential reproduction within a population that is correlated with the genotypes of individuals and the result of purposeful breeding choices by humans. An example is the purposeful mating among the longest-horned cattle in a herd to produce offspring that have horns that are longer than average.

Breeder's equation: (R = h² S) This equation describes the relationship among the response of selection (R), heritability (h²), and the selection coefficient (S). Traits with low heritability will show relatively low responses to a given level of selection pressure. In addition, the greater the selection coefficient, the higher the response (unless heritability is zero).

Deleterious trait: A characteristic of an organism that has negative consequences.

Diploid: Possessing two copies of every gene, one from each parent. Most familiar organisms are diploid.

Dominant allele: A particular version of a gene that is always expressed, no matter what other copy of the gene is possessed by the organism. An example of a dominant allele is the allele for black-colored hair in cattle (see page on coloration).

DNA: An abbreviation for deoxyribonucleic acid, the molecule that encodes the genetic instructions for almost all living organisms.

Environmental factors: Aspects of an organism's surroundings and experiences, such as diet and air temperature.

Fixed: In the context of the genetics of a population, an allele is said to be fixed if it is the only allele present at a given genetic locus. In other words, two copies of a fixed allele are present in all individuals of the population (in diploid organisms, such as cattle).

Gene: A nucleotide sequence on a chromosome that encodes a particular product (typically a protein).

Genetic linkage: The relative position of two genes on chromosomes within a genome. Two genes are said to be linked if they are on the same chromosome, and to be tightly linked if they are physically close to each other on the same chromosome. Two genes that are closely linked are usually inherited together from parent to offspring.

Genetic loci: Plural of genetic locus.

Genetic locus: A particular location on a chromosome (or on two homologous chromosomes in diploid organisms) that contains a functional gene.

Genotype: The genetic makeup of an organism (or a particular genetic combination). Often, geneticists speak of the genotype at a particular locus (= a gene), as in "That bull's genotype at the Extension locus is homozygous black."

Heritability: The proportion of the total phenotypic variation of a trait that has a genetic basis. Note that biologists often distinguish between narrow-sense heritability and broad-sense heritability. The difference is whether or not genetic variation is broken down into its additive, dominance, and epistatic components. Narrow-sense heritability is the proportion of the total phenotyic variation that has a genetic basis that results from the additive effects of genes, whereas broad-sense heritability is the proportion of the total phenotyic variation that has any kind of genetic basis. Non-additive genetic effects include dominance effects of alleles, as well as interactions among genes (epistasis). In practice, when we measure heritability as described on this web site, we are actually measuring heritability in the narrow sense, by assuming that the individual genes have additive effects.

Heterosis: The increased fitness of an organism that results from breeding between two different lines or populations. Heterosis is produced by increased heterozygosity.

Heterozygous: Possessing two different alleles at a given genetic locus across the two sets of homologous chromosomes.

Homozygous: Possessing the same allele at a given genetic locus across both sets of homologous chromosomes.

Inbreeding: Any mating system that increases the probability (above the expectations of an infinitely large, randomly mating population) that the two alleles of a gene in an organism will be inherited from the same copy of the gene in an ancestor. The most common factors that contribute to inbreeding are an excess of matings between close relatives and small population size.

Inbreeding depression: The loss of fitness that usually results from inbreeding. This loss of fitness is the result of increased homozygosity, which in turn exposes recessive deleterious traits.

Linebreeding: A form of purposeful inbreeding that involves mating among a restricted set of individuals. The set may be relatively large (e.g., Texas Longhorns), more restricted (e.g., the Butler family of Texas Longhorns), or highly restricted (e.g., relatives of a particular bull). As linebreeding becomes more restricted to smaller and smaller groups, the level of inbreeding increases, as does the chance for significant inbreeding depression. Inbreeding results in increased homozygosity across genetic loci, including genes that encode both deleterious as well as desirable traits. Linebreeding is used to perpetuate traits that are considered desirable, but often results in some level of inbreeding depression as well.

Loci: Plural of locus.

Locus: Short for genetic locus (see Genetic locus).

Natural selection: Differential reproduction within a population that is correlated with the genotypes of individuals and that is the result of the differential fitness of the genotypes. An example is selection for disease resistance in Texas Longhorns that lived in feral populations on the open range. Texas Longhorns that carried genes that conveyed resistance to disease were more likely to survive and reproduce compared to longhorns that lacked such genes. Natural selection is one of the primary (but not the only) mechanisms that result in biological evolution.

Nucleotide: One of the building blocks of DNA or RNA. There are four nucleotides in DNA: Adenine (A), cytosine (C), guanine (G), and thymine (T). These are the "letters" or "bases" of the genetic code.

Phenotype: The physical appearance of an organism.

Phenotypic variation: Variation in the physical appearance of an organism.

Qualitative trait: A characteristic of an organism that varies in a discrete, non-continuous manner (e.g., hair color of cattle occurs in discrete states, such as white, red, brown, and black).

Quantitative trait: A characteristic of an organism that varies in a continuous manner (e.g., horn length).

Recessive allele: A copy of a gene whose effect is only observed when it is homozygous.

Selection: Differential reproduction within a population that is correlated with the genotypes of individuals.

Trait: A characteristic of an organism.

Wild-type (as in Wild-type allele or Wild-type trait): The common or ancestral form of a gene.

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